High voltage silicon carbide (SiC) Schottky diodes, which may have voltage blocking ratings between, for example, about 600V and about 2.5 kV, are expected to compete with silicon PIN diodes having similar voltage ratings. Such diodes may handle as much as about 100 amps or more of forward current, depending on their active area design. High voltage Schottky diodes have a number of important applications, particularly in the field of power conditioning, distribution and control.
An important characteristic of a SiC Schottky diode in such applications is its switching speed. Silicon-based PIN devices typically exhibit relatively poor switching speeds. A silicon PIN diode may have a maximum switching speed of approximately 20 kHz, depending on its voltage rating. In contrast, silicon carbide-based Schottky devices are theoretically capable of much higher switching speeds, for example, in excess of about 100 times better than silicon. In addition, silicon carbide devices may be capable of handling a higher current density than silicon devices.
A conventional SiC Schottky diode structure has an n-type SiC substrate on which an n− epitaxial layer, which functions as a drift region, is formed. The device typically includes a Schottky contact formed directly on the n− layer. A junction termination region, such as a guard ring and/or p-type JTE (junction termination extension) region, is typically formed to surround the Schottky junction active region. The purpose of junction termination region is to reduce or prevent electric field crowding at the edges of the Schottky junction, and to reduce or prevent the depletion region from interacting with the surface of the device. Surface effects may cause the depletion region to spread unevenly, which may adversely affect the breakdown voltage of the device. Other termination techniques include field plates and floating field rings that may be more strongly influenced by surface effects. A channel stop region may also be formed by implantation of n-type dopants in order to prevent the depletion region from extending to the edge of the device.
Regardless of the type of termination used, the Schottky diode will fail if a large enough reverse voltage is applied to the junction. Such failures are generally catastrophic, and may damage or destroy the device. Furthermore, even before the junction has failed, a Schottky diode may experience large reverse leakage currents. In order to reduce such leakage currents, the junction barrier Schottky (JBS) diode was developed. JBS diodes are sometimes referred to as Merged PIN-Schottky (MPS) diodes. A conventional JBS diode 10 is illustrated in FIG. 1. As shown therein, a conventional JBS diode includes an n-type substrate 12 on which an n− drift layer 14 is formed. A plurality of p+ regions 16 are formed, typically by ion implantation, in the surface of the n− drift layer 14. A metal anode contact 18 is formed on the surface of the n− drift layer 14 in contact with both the n− drift layer 14 and the p+ regions 16. The anode contact 18 forms a Schottky junction with the exposed portions of the drift layer 14, and may form an ohmic contact with the p+ regions 16. A cathode contact 20 is formed on the substrate 12. Silicon carbide-based JBS diodes are described, for example, in U.S. Pat. Nos. 6,104,043 and 6,524,900.
In forward operation, the junction J1 between the anode contact 18 and the drift layer 14 turns on before the junction J2 between the p+ regions 16 and the drift layer 14. Thus, at low forward voltages, the device exhibits Schottky diode behavior. That is, current transport in the device is dominated by majority carriers (electrons) injected across the Schottky junction J1 at low forward voltages. As there may be no minority carrier injection (and thus no minority charge storage) in the device at normal operating voltages, JBS diodes have fast switching speeds characteristic of Schottky diodes.
Under reverse bias conditions, however, the depletion regions formed by the PN junctions J2 between the p+ regions 16 and the drift layer 14 expand to block reverse current through the device 10, protecting the Schottky junction J1 and limiting reverse leakage current in the device 10. Thus, in reverse bias, the JBS diode 10 behaves like a PIN diode. The voltage blocking ability of the device 10 is typically determined by the thickness and doping of the drift layer 14 and the design of the edge termination.
One problem associated with silicon carbide based Schottky diodes under forward bias operation arises due to the nature of the Schottky junction J1. Namely, the Schottky junction of a silicon carbide based device may have a relatively high resistance compared, for example to a PIN junction. In some power switching applications, current surges (e.g., transient current spikes) may be experienced from time to time. In Schottky devices, such current surges may result in the dissipation of large amounts of power at the junction, which results in heating of the junction. Heating of the Schottky junction may lower the barrier of the junction, resulting in even more current flowing through the device. This phenomenon, known as thermal runaway, may damage or destroy the device.
Thermal runaway may also occur in devices under reverse bias conditions, as reverse leakage currents may increase with temperature as a result of thermal runaway. Furthermore, other problems may arise in reverse bias conditions. For example, as noted above, if the blocking voltage of the device is exceeded, the device may break down in an uncontrolled manner, which may damage or destroy the device.